1. Field of the Invention
The present invention is directed to generating ultraviolet laser beams. More specifically, the present invention relates to a wavelength conversion method for converting wavelengths of laser beams with a wavelength conversion element made of a nonlinear optical crystal and a heating mechanism for heating and controlling the wavelength conversion element to a constant temperature.
2. Description of the Related Art
Recently, in such a wavelength conversion, a method in which a wavelength conversion element made of a single crystal of lithium tetraborate (Li2B4O7) having excellent chemical stability and resistance to laser damage is drawing attention. According to this method, based on the principle of second harmonic and the principle of sum frequency, it is possible to obtain various harmonics from different fundamental waves. For example, from the principle of second harmonic, second harmonic waves (green light having a wavelength of 532 nm) of an Nd:YAG laser are made to be incident to the wavelength conversion element such that fourth harmonic waves (ultraviolet light having a wavelength of 266 nm) are generated. Further, from the principle of sum frequency, the fundamental waves and the second harmonic waves of the Nd:YAG laser are made to be incident simultaneously to the wavelength conversion element such that third harmonic waves (ultraviolet light having a wavelength of 355 nm) are generated. Alternatively, the fundamental waves and the fourth harmonic waves of the Nd:YAG laser are made to be incident simultaneously to the wavelength conversion element such that fifth harmonic waves (ultraviolet light of a wavelength of 213 nm) are generated.
Also, since the nonlinear optical constant of the single crystal of lithium tetraborate is a small value of 0.16 pm/V, there is a drawback in that the conversion efficiency of the wavelength conversion is low. Thus, conventionally, incident light is condensed with a lens to generate high output ultraviolet laser such that the low conversion efficiency is compensated.
However, in the case in which the harmonics generated by the wavelength conversion with the wavelength conversion element made of the single crystal of lithium tetraborate have wavelengths of less than 355 nm (fourth and fifth harmonic waves), two-photon absorption of the single crystal of lithium tetraborate is caused, and heat is generated in the wavelength conversion element, which results in changed refractive index of the wavelength conversion element, such that phase matchability lowers. As a result, output loss or unstabilization is caused, and qualities of the laser beams deteriorates, which consequently results in a problem in that it is not suitable for a precise processing such as drilling of a printer board or scribing of various electronic components.